How do I update a package in Gentoo Linux?
This is what you need to read, an intro to portage, the package manager in Gentoo.
To update everything (including dependencies and changes to use flags), then
emerge -avDuN world
To update a single package (again paying attention to dependencies etc):
emerge -avDuN <package-name>
A basic update, without checking for dependency updates would be
emerge -avu <package-name>
-a means “ask” which is for confirmation, and the
-v means “verbose”, to give more info about what it is doing.